A fuel pump, which supplies high pressure fuel to an internal combustion engine via a common rail, is known in the art, for example, as disclosed in Japanese Patent Publication No. 2000-240531. Such a conventional fuel pump is shown in FIG. 6 attached to the present application. The fuel pump 100 has high pressure pump portions 101 for pressurizing fuel and pumping out such pressurized fuel, a cam mechanism 102 driven by the internal combustion engine to operate the high pressure pump portions 101, and a pump housing 103 for accommodating the cam mechanism 102 and holding the high pressure pump portions 101.
Each of the high pressure pump portions 101 is composed of a fuel pressurizing chamber 104 and a plunger 105 movably arranged in a cylindrical bore to expand or reduce the fuel pressurizing chamber 104, so that the fuel is sucked into the fuel pressurizing chamber 104 and pressurized fuel is pumped out from the fuel pressurizing chamber 104.
The cam mechanism 102 is composed of a shaft 106 driven to rotate by the engine, a cam member 107 integrally formed with the shaft 106 and a cam ring 108. The cam member 107 is eccentric with the shaft 106 and driven to rotate by the rotation of the shaft 106. An inner peripheral surface of the cam ring 108 is movably supported by an outer peripheral surface of the cam member 107, so that the cam ring 108 moves around the shaft 106 in accordance with the rotation of the cam member 107, without changing its posture.
A plunger head 109 having a larger diameter than that of the plunger 105 is integrally formed with the plunger 105 at its one axial end, which is on an opposite side of the fuel pressurizing chamber 104. The cam ring 108 has a contact surface 110, which is in contact with the plunger head 109. A coil spring 111 is provided for biasing the plunger head 109 toward the contact surface 110, so that the plunger head 109 is brought into contact with the contact surface 110. The plunger head 109 slides on the contact surface 110 so as to move relative to the cam ring 108, while the plunger head 109 is reciprocated in an axial direction of the plunger 105, in accordance with the rotation of the cam ring 108. A cam chamber 112 is formed in the housing 103 for accommodating the cam member 107, the cam ring 108, and the plunger head 109.
According to the above structure, since the fuel pressurizing chamber 104 is expanded and/or reduced in accordance with the reciprocal movement of the plunger 105, which is integrally formed with the plunger head 109, each of the high-pressure pump portions 101 draws the fuel into the fuel pressurizing chamber 104 and pressurizes the fuel to pump out the pressurized fuel from the fuel pressurizing chamber 104.
In recent years, various kinds of countermeasures have been taken into consideration so as to improve reliability of the fuel pump 100, supposing that inferior fuel including extraneous matters would be used, an abnormal high pressure would be generated and so on.
A countermeasure in view of a structure of the fuel pump is taken into consideration as one of countermeasures for improving the reliability, when supposing such a case in which the plunger head 109 would be separated from the plunger 105 as a result of use of the inferior fuel or generation of the abnormal high pressure.
In the case that the plunger head 109 was separated from the plunger 105, the relative movement of the plunger head 109 to the contact surface 110 ceases, and thereby the plunger head 109 would move together with the cam ring 108 and would be displaced in a direction other than the axial direction of the plunger 105. Then, an adverse and unexpected influence may be generated by the plunger head 109 against the pump housing 103.
It is, therefore, known in the art to provide a guide member for movably supporting the plunger head 109 in the axial direction of the plunger, in order to avoid a situation that the movement of the plunger head 109 relative to the contact surface 110 may be stopped. Namely, according to the above structure that the plunger head 109 is supported by the guide member, the plunger head can still relatively reciprocate on the contact surface 110, even when the plunger head 109 was separated from the plunger 105.
However, according to such a structure, a number of parts and components is inevitably increased. It is, therefore, desired to improve the reliability of the fuel pump without increasing the number of parts and components, supposing that the plunger head 109 would be separated from the plunger 105.